Heat exchangers of this kind usually have a generally cylindrical housing including a peripheral wall, centered on a vertical axis, whose ends are closed by a top and a bottom. The combustion gases, generated in the housing or admitted through its bottom, circulate in its interior around tubing traversed by the transfer fluid. The housing wall, manually made of steel, may be provided with a protective lining or heat shield of ceramic material, e.g. refractory bricks or tiles; this has the purpose of minimizing the loss of combustion heat due to thermal radiation.
Difficulties arise in such a structure because of the different thermal coefficients of expansion of the ceramic and metallic materials. Moreover, if the transfer fluid is an organic liquid such as a thermal oil, it is important to avoid any overheating thereof. This causes no problem during the operation of the heat exchanger. However, when the unit is turned off and the liquid no longer circulates, the heat stored in the ceramic materials may adversely affect the liquid still left in the tubing.